As a means for simultaneously improving color rendering properties and light output, the three peak emission system is known which uses blue, green and red light-emitting phosphors having relatively narrow light-emission spectra and mixed in a predetermined ratio. In a flourescent lamp of this type which uses a mixture of three types of phosphors emitting light of significantly different colors, misregistration of colors is caused by the difference in decreases in light outputs of the respective phosphors after some operation time of the lamp. The misregistration degrades the product quality of the lamp.
With recent development in office equipment, the number of copying machines used is increasing. Especially, copying machines using fluorescent lamps as light sources are prevailing so much in view of energy conservation. In a copying machine of this type, a decrease in the light output after some ON time of the fluorescent lamp as the light source gives rise to a decrease in the copying speed. For this reason, the decrease in the light output over ON time counts more than the initial light output. However, the flourescent lamps as light sources which are used under high load experience more decreases in light output than the general fluorescent lamps.
An yttrium silicate phosphor {(Ye, Ce, Tb).sub.2 O.sub.3.SiO.sub.2 } activated by cerium and terbium emits green light and has high luminous efficacy, and is therefore used as a fluorescent lamp of the three peak emission type or a flourescent lamp as a copying machine light source. Although the yttrium silicate phosphor has high luminous efficacy, it goes through a relatively great decrease in the light output after some ON time.
The present invention has been made in consideration of this and has for its object to provide a novel green-emitting phosphor in which a decrease in light output during ON time is reduced to the minimum.